This invention relates to a polymer processing additive that employs a tetrafluoroethylene copolymer and a poly(oxyalkylene) polymer, melt processable thermoplastic compositions that utilize this polymer processing additive, and methods of improving the melt processability of thermoplastic hydrocarbon polymers.
For any melt processable thermoplastic polymer composition, there exists a critical shear rate above which the surface of the extrudate becomes rough and below which the extrudate will be smooth. See, for example, R. F. Westover, Melt Extrusion, Encyclopedia of Polymer Science and Technology, Vol. 8, pp 573-81 (John Wiley and Sons 1968). The desire for a smooth extrudate surface competes, and must be optimized with respect to, the economic advantages of extruding a polymer composition at the fastest possible speed (i.e. at high shear rates).
Some of the various types of extrudate roughness and distortion observed in high and low density polyethylenes are described by A. Rudin, et al., Fluorocarbon Elastomer Aids Polyolefin Extrusion, Plastics Engineering, March 1986, at 63-66. The authors state that for a given set of processing conditions and die geometry, a critical shear stress exists above which polyolefins such as linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene suffer melt defects. At low shear rates, defects may take the form of xe2x80x9csharkskinxe2x80x9d, a loss of surface gloss, that in more serious manifestations appears as ridges running more or less transverse to the extrusion direction. At higher rates, the extrudate can undergo xe2x80x9ccontinuous melt fracturexe2x80x9d becoming grossly distorted. At rates lower than those at which continuous melt fracture is first observed, LLDPE and HDPE can also suffer from xe2x80x9ccyclic melt fracturexe2x80x9d, in which the extrudate surface varies from smooth to rough. The authors state further that lowering the shear stress by adjusting the processing conditions or changing the die configuration can avoid these defects to a limited extent, but not without creating an entirely new set of problems. For example, extrusion at a higher temperature can result in weaker bubble walls in tubular film extrusion, and a wider die gap can affect film orientation.
Certain fluorocarbon processing aids are known to partially alleviate melt defects in extrudable thermoplastic hydrocarbon polymers and allow for faster, more efficient extrusion. U.S. Pat. No. 3,125,547 to Blatz, for example, first described the use of fluorocarbon polymer process aids with melt-extrudable hydrocarbon polymers wherein the fluorinated polymers are homopolymers and copolymers of fluorinated olefins having an atomic fluorine to carbon ratio of at least 1:2 and wherein the fluorocarbon polymers have melt flow characteristics similar to that of the hydrocarbon polymers.
U.S. Pat. No. 4,904,735 (Chapman, Jr. et al.) describes a fluorinated processing aid for use with a difficultly melt-processable polymer comprising (1) a fluorocarbon copolymer which at the melt-processing temperature of the difficultly melt-processable polymer is either in a melted form if crystalline, or is above its glass transition temperature if amorphous, and (2) at least one tetrafluoroethylene homopolymer or copolymer of tetrafluoroethylene and at least one monomer copolymerizable therewith wherein the mole ratio is at least 1:1, and which is solid at the melt-processable temperature of the difficultly melt-processable polymer.
U.S. Pat. No. 5,397,897 to Morgan et al. Describes the use of copolymers of tetrafluoroethylene and hexafluoropropylene having high hexafluoropropylene content as processing aids in polyolefins.
U.S. Pat. No. 5,064,594 to Priester et al., and U.S. Pat. No. 5,132,368 to Chapman, Jr. et al. describe the use of certain fluoropolymer process aids containing functional polymer chain end groups including xe2x80x94COF, xe2x80x94SO2F, xe2x80x94SO2Cl, SO3M, xe2x80x94OSO3M, and xe2x80x94COOM, wherein M is hydrogen, a metal cation, or a quaternary ammonium cation for use with a difficultly melt-processable polymer.
U.S. Pat. No. 5,464,904 to Chapman et al. discloses the use of a copolymer of tetrafluoroethylene and hexafluoropropylene with a polyolefin. The copolymer is partially crystalline, contains no more than 2% by weight hydrogen, has a melt viscosity of from 0.1xc3x97103 to 20xc3x97103 Pa.s, and has an end of crystalline melting (Tm(end)) of from 170xc2x0 C. to 265xc2x0 C. The only enhancement of melt-processability described in this patent is shown in Example 25 where a concentration of 1000 ppm of the fluoropolymer in linear low density polyethylene is said to reduce the extrusion pressure of the extrudable composition. There is no showing of a reduction in melt defects.
U.S. Pat. Nos. 5,015,693 and 4,855,013 to Duchesne and Johnson disclose the use of a combination of a poly(oxyalkylene) polymer and a fluorocarbon polymer as a processing additive for thermoplastic hydrocarbon polymers. The poly(oxyalkylene) polymer and the fluorocarbon polymer are used at such relative concentrations and proportions as to reduce the occurrence of melt defects during extrusion. Generally the concentration of the fluoropolymer is present at a level of from 0.005 to 0.2 weight percent of the final extrudate and the poly(oxyalkylene) polymer is present at a level of from 0.01 to 0.8 weight percent of the final extrudate. Preferably, the weight of the fluorocarbon polymer in the extrudate and the weight of the poly(oxyalkylene) polymer in the extrudate are in a ratio of 1:1 to 1:10.
U.S. Pat. No. 5,527,858 to Blong and Lavallxc3xa9e discloses a melt processable fluoroplastic composition that comprises a melt processable fluoroplastic polymer and a minor amount (i.e., less than 20% by weight) of a poly(oxyalkylene) polymer. The fluoroplastic polymer comprises interpolymerized units derived from vinylidene fluoride and at least one ethylenically unsaturated copolymerizable, fluorinated monomer. By combining the poly(oxyalkylene) polymer with the fluoroplastic polymer, the fluoroplastic polymer can be melt processed at relatively low melt temperatures to form extrudates such as fuel line hoses or tubing. This is achieved without the need to modify the chemical structure of the fluoropolymer, to raise the melt-processing temperature, or to extrude at lower line speeds or shear rates.
It has been discovered that mixing a composition comprising a fluorothermoplastic polymer and a poly(oxyalkylene) polymer with a melt processable thermoplastic hydrocarbon polymer is surprisingly effective in reducing melt defects such as sharkskin in thermoplastic hydrocarbon polymers, especially polyolefins, or in postponing these defects to higher extrusion rates than can be typically achieved without the use of such a composition. These results are achieved without alteration of the chemical structure of the thermoplastic polymer and without the need for resorting to conventional methods of reducing melt defecting such as raising the melt-processing temperature or extruding at slower line speeds and lower shear rates. The use of this composition is also effective in reducing die buildup within an extruder.
Briefly, in one aspect, the present invention provides a melt-processable composition that comprises a major amount (i.e., at least 50% by weight) of a melt processable thermoplastic hydrocarbon polymer and a minor, but effective, amount of a processing additive that comprises (a) a fluorothermoplastic polymer containing interpolymerized units derived from (i) vinylidene fluoride, and (ii) at least two separate ethylenically unsaturated, copolymerizable fluorinated comonomers, and (iii) optionally at least one low molecular weight non-fluorinated alpha olefin comonomer, (i.e. ethylene or propylene), and (b) a poly(oxyalkylene) polymer.
In another aspect, the present invention provides a novel processing additive composition that comprises from about 50 to 80% by weight of the fluorothermoplastic polymer, and correspondingly from about 50 to 20 weight percent of the poly(oxyalkylene) polymer.
In yet another aspect, the present invention provides a method for improving the melt processability of a melt processable thermoplastic hydrocarbon resin by combining the thermoplastic hydrocarbon resin with an effective amount of the processing additive, blending the materials until there is preferably a uniform distribution of the processing additive in the thermoplastic hydrocarbon polymer, and melt processing the resulting blend.